In a general optical communication system using an intensity modulation/direct detection system, generally an output light power of a laser diode (semiconductor laser) changes with a current injected into the laser diode, so that an intensity modulated light is obtained by applying a modulated current pulse to the laser diode current-biased in the vicinity of an oscillation threshold value. Meanwhile, an I-L characteristic (i.e., a characteristic indicative of a relation between an injected current and an output light power) changes with temperature. Accordingly, in order to obtain a constant operating condition irrespective of an external temperature, it is necessary to drive the laser diode controlled to a constant temperature or carry out temperature compensation.
However, taking into consideration the fact that a change in I-L characteristic depending upon temperature cannot be easily specified, it is parctically difficult to carry out precise temperature compensation. Furthermore, it is not desirable from a viewpoint of deterioration of the laser diode to carry out temperature compensation only. Accordingly, precise temperature control of the laser diode is necessary in order to improve a reliability of the laser diode and eliminate a temperature compensating circuit.
Referring to FIG. 1, there is shown a conventional laser diode assembly including a laser diode chip 5 adapted to be controlled to a constant temperature by driving a Peltier device 3. The Peltier device 3 is mounted on a metal stem 2. The Peltier device 3 is provided to utilize a Peltier effect for cooling or the like such that flow of electric current through a contact point between different kinds of semiconductors causes generation or absorption of heat other than Joule heat at this contact point. A metal carrier 4 is mounted on the Peltier device 3, and the laser diode chip (which will be hereinafter referred to as LD chip) 5 and a thermistor 6 are mounted on the carrier 4.
The stem 2 is provided with a terminal 7 for connecting the thermistor 6 to an external circuit, a terminal 8 for connecting the Peltier device 3 to a driving circuit therefor, and a terminal 9 for connecting the LD chip 5 to a driving circuit therefor. These terminals 7, 8 and 9 extend through the stem 2. A common earth terminal 10 is connected to the stem 2. In this conventional laser diode assembly (which will be hereinafter referred to as LD assembly), a resistance of the thermistor 6 fixed on the carrier 4 is detected, and a driving current for the Peltier device 3 is so controlled as to maintain the resistance at a constant value. Accordingly, a heat quantity to be discharged from the LD chip 5 through the Peltier device 3 to the outside of the LD assembly is controlled to maintain a temperature of the LD chip 5 at a constant value. A cap 11 having a window 12 is tightly fixed to the stem 2 to sealingly enclose the inside of the LD assembly.
In the conventional LD assembly shown in FIG. 1, the terminal 7 and the thermistor 6 are connected together normally by a bonding wire 7a formed of gold having a good heat conductivity. As a result, in the case that an internal temperature of the LD assembly is different from an external temperature of the LD assembly, heat flows from the outside of the LD assembly through the terminal 7 and the bonding wire 7a into the thermistor 6, or heat flows from the thermistor 6 through the bonding wire 7a and the terminal 7 to the outside of the LD assembly, resulting in a problem that high-precision temperature control of the LD chip 5 cannot be carried out.
Referring to FIG. 2, reference numeral 13 designates the I-L characteristic in the case that the internal temperature of the LD assembly is equal to the external temperature of the LD assembly, wherein I.sub.th represents an oscillation threshold value of current. When the external temperature of the LD assembly becomes relatively high, heat flows from the outside of the LD assembly through the terminal 7 and the bonding wire 7a into the thermistor 6, so that a temperature of the thermistor 6 becomes higher than a temperature of the carrier 4 and the LD chip 5. Therefore, the temperature of the LD chip 5 is controlled to be lower than a desired temperature, and the I-L characteristic is shifted to the left as shown by reference numeral 14 in FIG. 2, resulting in a decrease in I.sub.th.
On the other hand, when the external temperature of the LD assembly becomes relatively low, heat flows out of the thermistor 6 through the bonding wire 7a and the terminal 7 to the outside of the LD assembly, so that the temperature of the thermistor 6 becomes lower than the temperature of the LD chip 5 and the carrier 4. Therefore, the temperature of the LD chip 5 is controlled to be higher than the desired temperature, and the I-L characteristic is shifted to the right as shown by reference numeral 15 in FIG. 2, resulting in an increase in I.sub.th. Such a change in I.sub.th due to a change in the external temperature is 2-3 mA, which is not ignorable in the case that the LD assembly is applied to an optical communication system of high speeds, e.g., about 1.8 Gb/s.